Merge rust-bitcoin#5351: CI: Use correct MSRV toolchain

3e4c121 CI: Use correct MSRV toolchain (Tobin C. Harding)

Pull request description:

  In commit 2a40a35 (PR rust-bitcoin#3418) I copied the `rust.yml` file from `master` and in doing so used the wrong MSRV toolchain.
  
  Use the correct MSRV toolchain for this branch. This means that since October 2024 we have not been testing in CI with the advertised MSRV toolchain.
  
  Fix: rust-bitcoin#5350


ACKs for top commit:
  apoelstra:
    ACK 3e4c121; successfully ran local tests; since MSRV for this branch won't ever change, but really we ought to somehow get a SSOT here


Tree-SHA512: 0f0c839cf345016e75462406bac4f34461aae086d3c9757388dac0844340a60e608e26170084ba60de83e2b73388aa3ab9c99d8e647a7d4df56eb34b5e289a58

Author: apoelstra

.github/workflows/rust.yml

@@ -88,7 +88,7 @@ jobs:
       - name: "Select toolchain"
         uses: dtolnay/rust-toolchain@stable
         with:
-          toolchain: "1.63.0"
+          toolchain: "1.56.1"
       - name: "Set dependencies"
         run: cp Cargo-${{ matrix.dep }}.lock Cargo.lock
       - name: "Run test script"